Composite materials made of a reinforcing fiber and a resin are widely used for sporting or leisure goods such as fishing rods and golf shafts, and equipment for airplanes, automobiles and industries. A molding material, referred to as prepreg, formed by impregnating a resin into a reinforcing fiber sheet made of reinforcing fiber bundles arranged in one direction, or a reinforcing fiber sheet made of reinforcing fibers arranged in at least two directions, for example a reinforcing fiber cloth, is used for molding of these composite materials. The composite material is produced by laminating a plurality of molding materials and molding the resulting laminate into a desired shape.
For production of the prepreg, in general, a resin sheet including a resin sheet formed of a release paper and a resin film formed from a resin coated the release paper, that is, a resin film with a release paper, and a carbon fiber bundle or a carbon fiber sheet including a carbon fiber bundle previously impregnated with a resin are used.
The surface of the resin film of the resin sheet is opposed to at least one surface of the carbon fiber sheet, and thereby a laminated sheet including the resin sheet and the carbon fiber sheet is formed. The resin in the formed laminated sheet is impregnated into the carbon fiber bundle in the laminated sheet under heating and/or pressure to produce a prepreg sheet including the carbon fiber bundle and the resin impregnated thereinto.
There are cases where two resin sheets are used and prepreg is produced by sandwiching the carbon fiber sheet between the two resin sheets.
As the resin, usually, an uncured thermosetting resin, for example, an uncured epoxy resin is used. The resin is coated the release paper by thin-film coating to form a resin film on the release paper. The thermosetting resin usually contains a curing agent. The thermosetting resin may be used in the form of a resin mixture including a thermoplastic resin mixed for the purpose of improving performance of a composite material or for the purpose of adjusting the viscosity of the resin in the production process.
As the carbon fiber bundle, a lot of carbon fibers arranged in one direction, or a lot of carbon fibers arranged in at least two directions, for example, a carbon fiber cloth, are used.
The impregnation of the resin into the carbon fiber bundle is usually conducted by a hot melt method. The impregnation of the resin into the carbon fiber bundle is performed once, or two or more times as required. The second or later impregnation of the resin into the carbon fiber bundle is performed for a carbon fiber bundle of prepreg formed by impregnating a resin into the carbon fiber bundle formed at the previous stage.
In the production process of the prepreg, defects due to foreign matter contamination that foreign matters such as cured products of a resin and fragments of a release paper are mixed in the prepreg, or due to the defective impregnation of a resin into the carbon fiber bundle (defective transfer of a resin to the carbon fiber bundle) may occur in the prepreg. Accordingly, it is important to minimize the occurrence of these defects in the production process of prepreg or detect the defects having occurred for producing composite materials of high quality which are free of defects or have little defects.
The detection of these defects of the prepreg is desirably performed without variation and with the defects classified by the type thereof according to criteria previously set in accordance with the type of the prepreg to be produced. Further, it is also desired that the position of the detected defect in the prepreg being continuously produced can be identified, and that the position of the defect can be marked on the prepreg. If the type of the defect can be identified and the position of the defect can be identified, reworking of the defect or elimination work of the defect area is enabled, and prepreg with high quality can be produced.
Conventionally, the detection of the defect of prepreg is generally performed by visual inspection. However, the visual work has problems that not only it requires extremely high concentration power, but also it tends to vary in judgment of the defect from person to person or some defects may be overlooked.
Introduction of a technique of automation of the prepreg defect inspection, which is aimed at solving these problems or saving manpower, into the production process of the prepreg is facilitated, and various techniques are proposed for shortening the inspection time or improving the inspection accuracy.
In Patent Document 1 is proposed a method of producing prepreg and an apparatus in which defects of the prepreg appearing in the production process are detected by classifying them into a fuzz defect, a gap defect and a foreign matter defect, and prepreg is produced by using the results of the detection. In this defect inspection method by Patent Document 1, the surface of the prepreg after the release paper is separated is irradiated with light to detect the defect in the surface of the prepreg. In this method, however, when a release paper is present on one surface of the prepreg, it is basically impossible to detect the defect occurred in the surface of the prepreg opposed to the release paper.
In Patent Document 2 is proposed an apparatus for producing prepreg and a method of producing prepreg in which a state of resin transfer (resin impregnation) to a reinforcing fiber bundle in the production process of the prepreg is detected, and prepreg is produced by using the results of the detection. In the inspection method of the state of resin transfer by Patent Document 2, after a resin coated a release paper is transferred to the reinforcing fiber bundle, remained resin on the surface of the release paper is inspected, and thereby, the state of resin transfer on the surface of the prepreg on a side from which the release paper has been separated is presumed. In this method, although the state related to resin transfer is detected, when a release paper is present on one surface of the prepreg, it is all the same basically impossible to detect the defect occurred in the surface of the prepreg opposed to the release paper.
In Patent Document 3 is proposed a method of inspecting defects of prepreg in which, among defects of prepreg, a gap defect and a fuzz defect are detected. However, a plurality of kinds of defects are present in the prepreg other than the gap defect and the fuzz defect, and therefore this method cannot completely save manpower and improve the inspection accuracy.
In Patent Document 4 is proposed a method of inspecting defects of prepreg in which, among defects of prepreg, a foreign matter defect, a fuzz defect, a streak defect and a wrinkle defect are detected. However, since a plurality of kinds of defects are present other than the foreign matter defect, the fuzz defect, the streak defect and the wrinkle defect, this method cannot completely save manpower and improve the inspection accuracy.
On the other hand, in the step of coating the release paper with a resin mixture to form a resin film on the release paper, the resin is partially lost on a coating roll to cause defects of resin coating that the resin is missed in the shape of a streak on the release paper, or the resin is missed in the shape of a point on the release paper.
Among these defects of resin coating, a streak-like defect may be referred to as a streak defect, and a spot defect may be referred to as a point defect. Further, foreign matters such as cured products of a resin and fragments of a release paper are sometimes mixed to cause a defect in a resin film before rolling-up a produced resin sheet. The defect due to foreign matters mixed may be referred to as a foreign matter defect.
These defects of a resin film may remain as defects in prepreg even after the resin is transferred to the reinforcing fiber bundle in the resin impregnation step of impregnating the resin into the reinforcing fiber bundle. Therefore, a criterion of the defect is defined for every kind of prepreg. Judgment of the defect is performed according to the criterion, and the position of the defect is recorded, and the defect is corrected or the defect area is eliminated from a product.
It is known to detect these defects in the production process of a resin film in which a resin film used for production of prepreg is formed.
In Patent Document 5 is proposed an apparatus for producing a resin sheet and a method of producing a resin sheet which detect a defect of a resin film formed by coating the surface of a release paper with a resin in the production process of a resin film, that is, a defect of resin coating. The defect inspection method described in Patent Document 5 is a method of detecting a defect of coating of a resin onto the surface of a resin film in the production process of a resin sheet.
However, in the production process of the resin sheet, even if a defect is detected before rolling up the resin sheet, it is basically impossible to detect a time-dependent change in the resin sheet before it is supplied to the subsequent production process of prepreg, a defect occurring in the rolling-up step of the resin sheet, or a defect occurring at the time of spreading a resin sheet from a resin sheet rolled body in the production process of prepreg, for example, a foreign matter defect occurring due to the mixing of the foreign matter.
Conventionally, usually the prepreg to which a release paper still adheres to one surface thereof, that is, prepreg with a release paper, is taken up in consideration of the handleability of prepreg when the prepreg is produced by a hot melt method. Therefore, the inspection of the defect on a surface of the prepreg on the side of the release paper is performed after separating the release paper. The release paper should be adhered to the prepreg in shipping the prepreg to a customer. However, the number of defects may be increased due to these operations, and insufficiently, only the inspection of the surface of the prepreg to which the release paper is not adhered is implemented. Accordingly, defects present on the opposite surface may be detected first upon use of the product, and remedy of this point is desired.